Water tanks used, for example, for pools or public bathhouses will be contaminated by swimmers or bathers because their sweat, nasal mucus, dirt, hair, etc. will be brought thereinto to cause the contamination. Although usually the water in the tanks is filtered to be purified by using a filter, the contaminants mentioned above can gradually accumulate in the filter, leading to a situation wherein bacteria grow in the filter. Elimination or sterilization of the bacteria thus grown in the filter is carried out, for example, by the supply of water to compensate the overflow or by the supply of fresh water, or by backwashing, or by charging with a chlorine sterilizer.
However, once the filter or the tank has become a site where bacteria inhabit, the bacteria are barely killed by supplying water or by charging with chlorine, and in addition the chlorine reacts with organic materials in the water to become combined chlorine, leading to the occurrence of organic materials in the filter.
Therefore, the inventor paid attention to the subject, and previously suggested an ozone supply apparatus (see Japanese Patent Application No. JP63-200891) suitable for sterilizing and purifying water. By circulating water in a water tank through the ozone supply apparatus, a more positive sterilization and purifying treatment became possible.